The present invention relates to a catalyst for stationary and/or fluid bed dehydrogenation processes for hydrocarbons, which is particularly useful in vapor phase dehydrogenation to produce lower olefins. The catalyst preferably comprises an alumina carrier, with at least, chromium oxide, sodium oxide and potassium oxide. The resultant catalyst exhibits higher conversion and selectivity and high olefin yields after the catalyst ages, low deactivation rates and robust hydrothermal stability in comparison to prior art catalysts.
Supported metal oxide catalysts are used in a variety of commercial reactions, and are typically present in the form of pellets or other shaped products or powders having active metal sites on or within an essentially chemically inert carrier. In many catalytic processes, a chemical reactant contained in a gas stream is passed over or through a bed containing the catalyst. The reactant contacts the active site on the catalyst, a chemical conversion occurs to generate one or more products, and those products are released from the catalyst's active sites. For commercial operations, it is desirable that the gas stream be passed over the catalyst bed at an essentially constant and rapid rate.
In the production of olefins and diolefins by catalytic dehydrogenation, it is desirable to obtain as high a yield of the desired olefins or diolefins as possible with high conversion and selectivity during a single passage of the material to be catalyzed through the dehydrogenation zone. It is also important to produce as small an amount of by-products and coke during the dehydrogenation process as is possible. Further, long catalyst life and low deactivation rates are important.
Selectivity plays an important role in olefin production. As the annual production of olefins (such as isobutylene, propylene and butadiene) by catalytic dehydrogenation is at least 3 million tons, even relatively small increases in selectivity of the catalyst, as small as a fraction of a percentage point, can be very beneficial financially for olefin producers.
One of the most important of the dehydrogenation processes is the Houdry paraffin dehydrogenation process that is conducted in a cyclic reaction mode. Cyclic processes make use of parallel reactors that contain a shallow bed of the preferred catalyst. The feed is preheated through a fixed heater before passing over the catalyst in the reactors. The hot product is then cooled, compressed and sent to the product fractionation and recovery station. To facilitate continuous operation, the parallel reactors are operated in staggered, timed cycles. Each complete cycle typically consists of dehydrogenation, regeneration, reduction and purge segments. In addition, to provide more favorable equilibrium conditions, the reactors are generally operated at sub-atmospheric pressures during the dehydrogenation cycle. The regeneration cycle, operating in the range of 500 to 700° C., provides heat for the subsequent dehydrogenation reaction. Because of such high temperatures, the effective life of the dehydrogenation catalyst is generally no longer than about two to three years. After such periods of time on line, catalyst replacement is required because of reduced levels of conversion and selectivity. For instance, after two to three years of operation, the catalyst's conversion is generally reduced by 5 to 15% and its selectivity drops by about 5 to 20%. Thus, improvement in the catalyst's conversion and selectivity toward the end of a two to three year cycle can significantly improve the overall process efficiency.
While it is common to optimize a catalyst's performance based on its initial conversion and selectivity, it is also important to optimize catalyst performance based on its conversion and selectivity after aging.
Processes utilizing chromia-alumina catalysts for the conversion of paraffinic and olefinic hydrocarbons are well known and have been described in technical literature as well as in numerous patents starting in the 1940's. One typical composition for catalysts that are used for dehydrogenation of paraffins and olefins contains chromium oxide on the surface of an aluminum oxide carrier. Although chromia-alumina catalysts have a relatively high dehydrogenation activity, they often suffer from rapid coke formation during the dehydrogenation reaction. Consequently, frequent high temperature regeneration cycles are mandated. Because of these frequent regeneration cycles, it is required that the chromia-alumina catalyst have a high degree of hydrothermal stability in order to extend the life of the catalyst.
Other types of dehydrogenation catalysts contain platinum, palladium or other precious metals on various carriers, including alumina carriers. However, while both platinum-based and chromium-based dehydrogenation catalysts are used commercially, the nature and behavior of these two catalyst types are quite different.
For example, after regeneration, the small Pt particles on the surface of the platinum-based dehydrogenation catalysts become agglomerated, which causes a significant loss of activity. To return these catalysts to an active state, the Pt particles are subsequently re-dispersed by treatment with chloride. In contrast, chromium-based dehydrogenation catalysts require no such chloride treatment. In fact, chlorides are a severe poison for chromium-based dehydrogenation catalysts and must therefore be limited to no more than 1.0 ppmw in the form of HCl in the feed.
On the other hand, sulfur is a severe poison to platinum-based dehydrogenation catalysts, but chromium-based dehydrogenation catalysts can tolerate sulfur levels as high as 100 ppmw with little or no impact on performance.
These two types of dehydrogenation catalysts are also quite different from a compositional standpoint. In the case of platinum-based dehydrogenation catalysts, the active dehydrogenation component (Pt) is typically present in an amount of less than 1 wt %. In addition, these platinum-based catalysts require an element from Group IV as a promoter. They also typically contain a significant amount of a halogen (up to 1 wt %). In contrast, chromium-based dehydrogenation catalysts contain the active dehydrogenation component (Cr2O3) at higher percentage levels, typically 10-30 wt %, require no group IV element, and must not contain any halogen, as it is a poison.
These two types of dehydrogenation catalysts are also quite different from an operational standpoint. Platinum-based dehydrogenation catalysts require the hydrocarbon feed to be diluted with hydrogen and/or steam while chromium-based dehydrogenation catalysts require no hydrogen dilution with the feed and water is a poison for the catalyst.
Accordingly, a person skilled in the art would not anticipate that a change in composition of a platinum-based dehydrogenation catalyst which resulted in improved performance would necessarily cause a similar improvement in performance for a chromium-based dehydrogenation catalyst.
Additional components are often added to dehydrogenation catalysts to enhance their reactivity or selectivity or to enhance the operating characteristics of the carrier. One common additive is a single alkali metal or alkaline earth metal. Numerous patents have described the addition of an alkali metal to dehydrogenation catalysts, wherein the alkali metal of choice generally includes any of the alkali metals from lithium to cesium. Sodium or potassium compounds are often chosen for use because of their low cost and simplicity of utilization. It has generally been assumed that there is no significant difference in performance of the catalysts regardless of the choice of the alkali metal promoter although some references have recommended one alkali metal over other alkali metals.
There have also been some disclosures of a combination of alkali metals being utilized for chromium-based dehydrogenation catalysts. For example, U.S. Pat. No. 7,012,038 discloses a dehydrogenation catalyst which requires the addition of both lithium oxide and sodium oxide to a chromium oxide/alumina catalyst.
In addition, the use of lithium and potassium to a platinum-alumina dehydrogenation catalyst is disclosed in U.S. Pat. No. 4,677,237.
Notwithstanding the prior art, additional improved chromia/alumina catalysts with enhanced conversion and selectivity, especially as the catalyst ages, are still needed.
It is one object of the invention to produce a useful dehydrogenation catalyst comprising chromia distributed on an alumina carrier, which is promoted with both sodium oxide and potassium oxide. The use of sodium oxide and potassium oxide together as promoters provides enhanced performance for this inventive catalyst over prior art dehydrogenation catalysts, although this phenomenon primarily occurs after aging, wherein improvements in conversion and selectivity are particularly noted.
It is a further object of the invention to disclose useful processes for the preparation of the improved dehydrogenation catalysts comprising chromia deposited on a alumina carrier, which is promoted with both sodium oxide and potassium oxide.
It is a further object of the invention to disclose an improved process for dehydrogenating hydrocarbons, particularly lower paraffins, especially after the aging of the catalyst, wherein an improved dehydrogenation catalyst is utilized comprising chromia deposited on an alumina carrier, which is promoted with both sodium oxide and potassium oxide.
These and other objects of the invention can be obtained by the catalysts, the processes for preparation of the catalysts and the processes for dehydrogenation of hydrocarbons, particularly lower paraffins, which are disclosed herein.